Add internal DiversifiedTransmissionKey type

src/address.rs

@@ -1,6 +1,4 @@
-use halo2::pasta::pallas;
-
-use crate::keys::Diversifier;
+use crate::keys::{DiversifiedTransmissionKey, Diversifier};
 
 /// A shielded payment address.
 ///
@@ -15,11 +13,11 @@ use crate::keys::Diversifier;
 #[derive(Debug)]
 pub struct Address {
     d: Diversifier,
-    pk_d: pallas::Point,
+    pk_d: DiversifiedTransmissionKey,
 }
 
 impl Address {
-    pub(crate) fn from_parts(d: Diversifier, pk_d: pallas::Point) -> Self {
+    pub(crate) fn from_parts(d: Diversifier, pk_d: DiversifiedTransmissionKey) -> Self {
         Address { d, pk_d }
     }
 }

src/keys.rs

@@ -214,8 +214,8 @@ impl From<&FullViewingKey> for IncomingViewingKey {
 impl IncomingViewingKey {
     /// Returns the payment address for this key corresponding to the given diversifier.
     pub fn address(&self, d: Diversifier) -> Address {
-        let g_d = diversify_hash(&d.0);
-        Address::from_parts(d, ka_orchard(&self.0, &g_d))
+        let pk_d = DiversifiedTransmissionKey::derive(self, &d);
+        Address::from_parts(d, pk_d)
     }
 }
 
@@ -232,3 +232,17 @@ impl From<&FullViewingKey> for OutgoingViewingKey {
         fvk.derive_dk_ovk().1
     }
 }
+
+/// The diversified transmission key for a given payment address.
+#[derive(Debug)]
+pub(crate) struct DiversifiedTransmissionKey(pallas::Point);
+
+impl DiversifiedTransmissionKey {
+    /// Defined in [Zcash Protocol Spec § 4.2.3: Orchard Key Components][§4.2.3].
+    ///
+    /// [§4.2.3]: https://zips.z.cash/protocol/nu5.pdf#orchardkeycomponents
+    fn derive(ivk: &IncomingViewingKey, d: &Diversifier) -> Self {
+        let g_d = diversify_hash(&d.0);
+        DiversifiedTransmissionKey(ka_orchard(&ivk.0, &g_d))
+    }
+}